1. Field of the Invention
This invention relates to a magnetic flux scanning method and apparatus for examining the grain structure of an electrically conductive metallic workpiece and, more particularly to the use of a magnetic field for detecting sites where there are a concentration of small grains in an electrical steel sheet product, such as a silicon steel.
2. Description of the Prior Art
The desired electrical properties for electrical steels are the result of a special manufacturing techniques including melting, rolling and heat treating procedures to establish relatively large grains with a very little dispersion of small grains in the final texture annealed silicon steel strip. Specific grain structure is developed and refined to impart desired electrical properties to the steel strip. Conventional grain oriented silicon steel has a typical grain size on the order of 3 to 5 mm and high permeability grain oriented steel on the order of 10-15 mm. Sometimes occurrences, such as inclusions of impurities or rolling and heat treating process deviations, may allow non-uniform grains to develop across the width and length of the strip. In particular, for example, a concentration of small grains or a mixture of large grains with many small grains and/or orientation of grains exhibit magnetic flux boundaries caused by the grain boundaries. As a result, there exists unwanted flux barriers that downgrade the intended electric properties. Not only may the grain size vary appreciably, but also there are poor qualities of material that appear as coarsened primary grains known as "banding". It is desirable to locate and accurately identify the unacceptable sites of banding and mixed grains structure so that bad material can be sheared, cut or otherwise extracted from the metal strip.
In U.S. Pat. No. 4,079,312 there is disclosed a testing apparatus for determining magnetic characteristics for a strip of moving material. A flux inducing and pick-up device is used for measurement of the flux induced into the strip of moving material. A magnetization coil is used to induce magnetic flux into the strip, two flux pick-up coils are arranged to respond to different flux characteristics at about the same area of the strip. Voltage outputs corresponding to the two different characteristics are processed in an electrical circuit to derive an output signal according to a mathematical formula. Such a continuous strip testing device is well suited to identify that poor quality magnetic properties of the strip exist within a broad area. However, this testing procedure can not be used to identify the specific site or nature of the deficient magnetic properties. Particularly important is the fact that the device can not differentiate between banding or mixed grain structures in silicon steel. It has been found that for silicon steel strip, banding and mixed grain structure can occur in very small sections, but those sections may be known to exist only generally within a broad area of a strip through the use of the device shown in the aforesaid U.S. Pat. No. 4,079,312.
Others have attempted to develop an on-line grain size measuring system for electrical steel production. Particularly, an ultrasonic device is suggested in U.S. Pat. No. 4,893,510 to measure a distribution of crystal grains in a metal sheet. An estimate is made of grains orientation based on the estimate of the magnitudes of amplitude of interferant multiple reflected waves. The frequency of bursts of ultrasonic pulses is related to the sheet-thickness and applied in the direction of the sheet thickness to obtain the reflected waves.
What is needed is a device that can locate and identify irregularities that exist in the grain structure on a continuous basis on a production line at strip speeds of 100 to 450 feet per minute (30.48 to 137.16 meters/min.). It is also desirable that such a device be useful in conjunction with other continuous strip testers, such as for measuring magnetic properties, to better understand the irregularities in the grain structure and the quality of the electrical sheet product.
Therefore an object of the present invention is to provide a flux scanning procedure and device to locate, identify the severity, and differentiate between banding and mixed grain structures in a grain oriented silicon steel strip.